1. Field of the Invention
This invention relates to the treatment of wastewater, namely a method for reducing nitrate concentration in wastewater.
2. Background
Wastewater is treated to remove its bacterial nutrients, principally biochemical oxygen demand (BOD). BOD is degraded by bacteria primarily by consuming dissolved oxygen (DO) to assist in metabolizing the BOD (aerobic metabolism). Oxygen is provided in treatment systems by aeration. In an oxygen depleted environment when DO levels in the wastewater are sufficiently reduced (anoxic environment), bacteria readily metabolizes nitrates in the wastewater. Nitrate reduction by bacteria, however, is generally inefficient in the presence of oxygen. Bacteria capable of functioning in oxygen-rich and oxygen-poor environments will preferably metabolize nutrients aerobically.
If an anoxic environment is maintained for a sufficient time period, nitrate levels within the wastewater can be substantially reduced because bacteria will be forced to consume nitrates (hereinafter referred to as nitrate metabolism) as a food source resulting in reduced nitrate concentrations in the effluent (outgoing water). This process is referred to as de-nitrification. Note that minor de-nitrification may occur in an aerobic system if pockets of the treatment system are isolated due to poor current flow (hence poor oxygen flow) within the system. Additionally, in large bacteria or dense bacteria colonies, de-nitrification can occur in an aerobic environment. In large bacteria, oxygen transport in the cell may be inefficient resulting in interior parts of the cell undergoing anaerobic metabolism or nitrate metabolism. In large bacteria colonies, bacteria buried in the colony may not be exposed to the same levels of oxygen as exposed portions of the bacterial mass) and hence portions of these buried masses may undergo nitrate metabolism. A buried bacterial mass is more likely to occur in a fixed film treatment process. These instances of nitrate metabolism are considered insubstantial, and are insufficient to substantially reduce nitrate levels as required by new treatment standards.
New environmental regulations are requiring reduction in nitrate levels in the effluent from wastewater treatment systems. Several methods, such as those disclosed in U.S. Pat. Nos. 5,582,734 and 4,663,044, discuss possible solutions to nitrate removal in large municipal and commercial wastewater treatment plants. These solutions are inapplicable to relatively small, self-contained wastewater treatment systems that are employed throughout the United States for the treatment of wastewater. The small self-contained systems serve wastewater sources that include single family dwellings located in areas not served by municipal sewerage systems. Like larger commercial and municipal treatment systems, these smaller systems are becoming subjected to stricter nitrate removal regulations and encounter treatment problems when attempting to reduce nitrate levels in the wastewater. Unfortunately, no practical solution has been provided that will effectively solve the problems encountered by the small, self-contained systems.